Does NSNotificationCenter notications have higher priority than UITableView cell loading events?

Question

Are events posted by NSNotificationCenter postNotificationName processed before UI updating events?

I need to know because otherwise my current program will crash in some rare cases.

Model code:

- (void)searchFinishedWithResults:(Results *)results {
    self.results = results;
    // If some table cells are loaded NOW, before notication is processed, we might crash!
    [[NSNotificationCenter defaultCenter]
     postNotificationName:SearchResultArrived object:nil];    
}

When processing the notication, I will run UITableView reloadData.

However, consider if before processing the notication, UI has to be updated. In this case -tableView:cellForRowAtIndexPath:indexPath will be called, but results object has changed, it will fetch old data.

Answer 1

The notifications are dispatched exactly when you call postNotification: or postNotificationName:object:, in a synchronous fashion, one observer after the other (in no particular order). In the case you show, they would be sent exactly after you assign the variable "results" and before the method ends.

Answer 2

Directly from Apple's documentation on NSNotificationCenter:

A notification center delivers notifications to observers synchronously. In other words, the postNotification: methods do not return until all observers have received and processed the notification.

To send notifications asynchronously use NSNotificationQueue.